International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XXXIX-B4, 2012 
XXII ISPRS Congress, 25 August — 01 September 2012, Melbourne, Australia 
    
   
DTM CROSS VALIDATION AND MERGING: PROBLEMS AND SOLUTIONS FOR A 
CASE STUDY WITHIN THE HELI-DEM PROJECT 
Ludovico Biagi *, Laura Carcano *”, Mattia De Agostino b 
* Politecnico di Milano — DIIAR, Geomatics Laboratory at Como Campus, Via Valleggio 11, IT-22100 Como, 
*(laura.carcano @mail.polimi.it) 
^ Politecnico di Torino — DIATI, Corso Duca degli Abruzzi 24, IT-10129 Torino 
KEY WORDS: DEM/DTM, Georeferencing, Multiresolution, Accuracy, Algorithms, Correlation 
ABSTRACT: 
Almost all official national cartographic bases in Europe include Digital Terrain Models (DTMs). To perform analyses at the borders 
between countries, transnational geographic information is needed: a transnational DTM can be obtained by merging regional DTMs 
(usually at low resolution) and other typically local high resolution DTMs. To be used the different DTMs must be in the same three 
dimensional reference frame, they must not present biases and must be consistent at the nominal scale. The following preliminary 
actions are required: 
e  3Dreference frame transformation of gridded data, 
®  cross-validation of the regional DTMs, 
e  cross-check of the regional LR DTMs and the local HR DTMs. 
The paper discusses the study, the implementation and the application of algorithms to solve these problems in the Alpine area 
between Italy and Switzerland. For this case study, the regional DTMs of Lombardy, Piedmont and Switzerland, respectively with a 
planimetric resolution of 20, 50 and 25 meters are available. In addition, a LiDAR DTM with a planimetric resolution of 1 meter for 
the Lombardy and Piedmont hydrographic basins is available. The different DTMs have been transformed to the same reference 
frame by applying the official national 3D transformations. The cross-validation and the cross-check of the elevation datasets have 
been performed. All these operations have required the implementation of novel approaches to interpolate and compare DTMs and to 
spatially analyze their differences. 
1. INTRODUCTION 
Elevation data are fundamental in Geographical Information 
Systems (GIS) and one of the spatial data provided by the 
European Directive INSPIRE (Infrastructure for Spatial 
Information in Europe). They are stored in Digital Elevation 
Models (DEM, Li et al., 2005, El-Sheimy et al., 2005), which 
are the basic tools for many environmental and territorial 
applications. DEM refers to the generic family of elevation 
models: they are distinguished between Digital Surface Models 
(DSMs), which represent the surface (including buildings, 
woods, etc. and Digital Terrain Models (DTMs) which 
represent the elevation of bare soil. DTMs are important in 
Environmental Engineering, Civil Engineering and in many 
specific applications, such as the design of telecommunication 
networks or precise agriculture. 
In recent years, both in Italy and Switzerland, elevation data 
with different reference frames, technologies, accuracies and 
data models have been acquired. HELI-DEM (HEL vetia-Italy 
Digital Elevation Model) project (Biagi et al., 2011), funded by 
the European Regional Development Fund (ERDF) within the 
Italy-Switzerland cooperation program, aims at developing a 
unique DTM for the alpine and subalpine area between Italy 
(Piedmont, Lombardy) and Switzerland (Ticino and Grigioni 
Cantons) The DTM will be correctly georeferenced and 
produced validating and integrating all the available data. To 
meet this task with a planimetric resolution equal or better than 
20 meters and the best achievable accuracy according to the 
available products, a recomputation of the geoid, i.e. the 
elevation datum, with an accuracy better than 10 cm is required 
(Barzaghi et al, 2007, Marti et al, 2007). Also the 
determination of a GNSS network for the validation and 
georeferencing along the border is necessary. The institutions 
involved in the project are: Fondazione Politecnico di Milano, 
Politecnico di Milano, Politecnico di Torino, Provincia 
Autonoma di Bolzano, Regione Lombardia, Regione Piemonte 
and SUPSI (Scuola Universitaria della Svizzera Italiana). Two 
other institutions, SwissTopo and the Military Geographic 
Institute (IGM) are involved as external partners. The project is 
divided into nine Work Packages (WP) whose title and duration 
are shown in Table 1. 
In the paper, the first steps of the project will be discussed: in 
Sect. 2 the available data are presented, in Sect. 3 the adopted 
strategies to cross-check the data are discussed and their 
application to some case studies are reported. 
  
Initial | Final 
  
  
  
  
  
WP | Title Month | Month 
WP1 | Management 1 30 
Collection and analysis of the 
WP2 | available data (DEMs and 1 6 
geoid) 
WP3 Cross validation of DTMs 5 16 
and geoidss 
WP4 | GNSS permanent networks 7 18 
Height datum (geoid) 
WPS | unification between 12 23 
Switzerland and Italy 
External validation and 
Wea georeferencing of DEMs A 26 
Integration of all validated 
  
  
  
  
WP height data le 27 
Experimental use of the 
WP8 | project results and final 22 27 
evaluation of the data 
WP9 Diffusion and promotion of 19 30 
  
  
  
  
  
the research results 
  
Table 1. Work Packages of the HELI-DEM projects